that presents major challenge to law enforcement. abc's david kerley has more on this. >> reporter: the bloody boston explosion, shattered the marathon and reopened wound in this country. this attack far from the first targeting civilians. international terrorists tried to bring down the trade center in '83. the blast withstood and six were killed. the oklahoma city bombing in '95 killing 168. so similar to the boston blast, the atlanta olympics bombing in '96. spectators gathered in a park when a bomb went off. it is the soft target terrorists are looking for. >> these setting, marathon, football, football, social event, are targets. you can't get around it. >> reporter: tom ridge was the country's first secretary of homeland security. >> you have to manage the risk. unless you have information about a particular actor. you can't guarantee safety anywhere. >> though police are in duty with plenty of national guard soldiers some one was able to place two gomz. >> what can we do if we are at a large event is there anything i can do to